System and method for supporting split transactions on a bus

ABSTRACT

System and method for supporting split transactions on a bus. The method may comprise processing a periodic frame list of external bus data frame by frame, and traversing each frame node by node. When a save place node is encountered in a first frame, the traversing jumps to a destination node pointed to by the save place node in a second frame, and continues the traversing there. When a restore place node is encountered when traversing the nodes in the second frame, the traversing returns to the node after the save place node in the first frame and continues the processing in the first frame. The method may be implemented on a system that comprises a processor, a memory, an internal bus, and an external bus controller. The external bus controller and the external bus data may support one or more versions of the Universal Serial Bus standard.

FIELD OF THE INVENTION

[0001] This invention relates to computers and computing devices that include a bus controller for an external bus such as the Universal Serial Bus (USB), and more particularly to providing support in the bus controller for improved throughput on the external bus when supporting devices which adhere to both newer and older versions of the external bus protocol.

BACKGROUND

[0002] As personal computers have evolved, the processing power and available memory have both increased with each passing year. However, to allow for continuing use of existing software applications and hardware devices, current computers must take into consideration limitations inherent in earlier computer models and earlier external devices so as to provide compatibility. This is sometimes referred to as legacy support, support for legacy systems, support for legacy devices, etc.

[0003] An external bus supported by many current personal computers is the Universal Serial Bus (USB). Version 1.1 of the USB standard provides for low speed communication at 1.5 Mbps and full speed communication at 12 Mbps. For more information see Universal Serial Bus Specification, Revision 1.1 dated Sep. 23, 1998 available from USB Implementers Forum, Inc. of Portland, Oreg. A newer version of the USB standard, version 2.0, provides for high speed communication at 480 Mbps., as well as providing support for low and full speed communication. For more information see Universal Serial Bus Specification, Revision 2.0 dated Apr. 27, 2000 available from USB Implementers Forum, Inc. of Portland, Oreg. Because many existing external USB enabled devices such as digital cameras, printers, etc., adhere to the USB 1.1 standard, USB 2.0 takes this into consideration and allows for USB 1.1 devices to be used with USB 2.0 host controllers and hubs.

[0004] To accomplish the improvement in speed over USB 1.1, USB 2.0 defines micro-frames of 125 microseconds such that eight micro-frames comprise the 1 millisecond frames described in the USB 1.1 standard. As each frame includes several micro-frames under USB 2.0, each frame may contain several transactions. For isochronous and other kinds of periodic transfers which initiate from a USB 1.1 device, multiple transactions may be split and may be included in micro-frames in each of a plurality of successive frames. It is up to the USB 2.0 host controller and hub controller implementation to support split transactions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The invention described herein is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one.

[0006]FIG. 1 illustrates a hardware environment in which one embodiment of the invention executes.

[0007]FIG. 2 illustrates a diagram of various data locations and data structures involved with an embodiment of the invention.

[0008]FIG. 3A illustrates a link pointer according to one embodiment of the invention.

[0009]FIG. 3B illustrates a frame span traversal node according to one embodiment of the invention.

[0010]FIG. 4 illustrates an example sequence of nodes traversed during one frame according to an embodiment of the invention.

[0011]FIG. 5 illustrates an example sequence of nodes traversed during one frame according to an embodiment of the invention.

[0012]FIG. 6 illustrates a flow of actions taken pursuant to an embodiment of the invention.

DETAILED DESCRIPTION

[0013] The Universal Serial Bus (USB) is an ever-popular external bus standard that may be often used for connecting external devices to personal computing devices such as personal computers. Version 1.1 of the USB standard provides for low speed communication at 1.5 Mbps and full speed communication at 12 Mbps, while a more recent version of the USB standard, version 2.0, provides for high speed communication at 480 Mbps., while also supporting low and full speed communication. See Universal Serial Bus Specification Revision 1.1 dated Sep. 23, 1998 and Universal Serial Bus Specification, Revision 2.0 dated Apr. 27, 2000 both available from USB Implementers Forum, Inc. of Portland, Oreg. These two standards will be referred to as USB 1.1 and USB 2.0 respectively, and collectively as the USB standards. Although USB 2.0 provides for splitting or staggering USB 1.0 transactions across multiple frames, no implementation of how to manage and store USB data is provided in the USB standards. However, the Enhanced Host Controller Interface Specification for Universal Serial Bus, revision 0.96 dated Jun. 20, 2001 available from Intel Corporation of Santa Clara, Calif. (the EHCI Specification) provides a standard for the handling of USB data. The various embodiments of the invention described herein provide a system and method for traversing USB data that includes the splitting of USB transactions over multiple frames in a manner which increases efficiency and throughput of communications between USB devices and a USB 2.0 host controller. The various embodiments of the invention described herein include information from the EHCI Specification, pertinent portions of which are attached as an Appendix.

[0014]FIG. 1 illustrates a hardware environment in which one embodiment of the invention executes. A computing device such as personal computer 100 may include processor 102, memory 104, storage device 106, graphics adapter 110 and USB host controller 120 coupled with bus 140. Processor 102 may be any kind of processor and memory 104 may be any kind of random access memory (RAM). Personal computer 100 may include graphics adapter 110 that allows for the display of information such as text and graphics on display monitor 112.

[0015] In one embodiment, storage device 106 may be any kind of machine readable medium including, for example, magnetic media such as disk drives and magnetic tape; optical drives such as compact disk read only memory (CD-ROM) and readable and writeable compact disks (CD-RW); stick and card memory devices; read only memory (ROM), RAM, flash memory devices and the like; whether internal, such as storage device 106, directly coupled, accessible locally or accessible remotely via a network, via electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.) via a communications device (not shown). In one embodiment, operating system (OS) 114 and drivers 116 may be stored on storage device 106. Drivers 116 serve as the interface between operating system 114 and the various hardware components included in personal computer 100, including memory 104, graphics adapter 110 and USB host controller 120. For each hardware component included in personal computer 100, a driver for that component is stored on storage device 106.

[0016] Personal computer 100 may include a Universal Serial Bus (USB) host controller 120 which allows for two-way communication with external USB devices. Various USB enabled input devices may be coupled to personal computer 100 via USB controller 120, such as, for example, keyboard 132 and mouse 134. Other external devices which may be coupled to USB host controller 120 include, for example, camera 136, scanner 138, MP3 players, printers and additional USB enabled input devices such as a pen and tablet and a trackball. Although only one USB host controller is shown, multiple USB host controllers may be included in personal computer 100. In addition, each of the USB enabled devices may serve as a hub for other USB devices, and external hubs may be coupled to USB host controller 120.

[0017] In one embodiment, USB host controller 120 includes a state machine which manages the traversal of USB data stored as USB data 130 in memory 104 by a USB driver. This state machine may be referred to as host controller state machine (HCSM) 124. The hardware of USB host controller 120 may also include registers 126 and flags 128. In one embodiment, the flags may be included in a flag register such that the flag register is one of the registers included in the USB host controller.

[0018] In the various embodiments of the system and method presented herein, the method may be implemented as HCSM 124 which resides in USB host controller 120. In another embodiment, the HCSM may be implemented as software which may be executed on a small processor included in host controller 120 (not shown). In this software embodiment, the software could be stored on storage device 106 and transferred to host controller 120 for execution thereon, or the software may be stored on a programmable read-only memory (PROM) or similar device included on host controller 120 (not shown).

[0019] During start-up of personal computer 100, a USB host controller driver (included in drivers 116) obtains access to a portion of memory 104 and uses it to store USB data 130. USB data 130 includes various data structures constructed according to an implementation supporting USB 2.0. HCSM 124 then manages the traversal of and processing of the data structures and data included in USB data 130.

[0020]FIG. 2 illustrates a diagram of various data locations and data structures involved with an embodiment of the invention. In one embodiment, USB host controller 120 may include a plurality of registers 126. Registers 126 may include a frame list base address register 212 and a frame index register 214. Frame list base address register 212 may be used to store a frame list base address which may point to the location in memory 104 where a periodic frame list 232 of USB data 130 is located. The address of a current frame to be processed is designated as an offset known as frame index stored in frame index register 214. The registers may also include a return node address pointer register 216, a current node pointer register 218, a next node pointer register 220, and flags register 128. Among other flags, flags register 128 may include recovery mode flag 224. These registers and flags are discussed in more detail below.

[0021] The location in memory 104 of the current frame list head pointer of periodic frame list 232 is determined by combining the frame list base address with the frame index. In one embodiment, the periodic frame list is a 4 Kbyte page aligned array of frame list link pointers referred to as frame list head pointers 234. In one embodiment, each of the frame list link pointers is a 32 bit double word. In various embodiments, the periodic frame list may be of any desirable size, and the size of the periodic frame list may be programmable by driver 116 to any of the sizes supported by USB controller 120. In one embodiment, the periodic frame list may contain 1024 frame list link pointers. In other embodiments, the periodic frame list may contain 256, 512, etc. frame list link pointers.

[0022]FIG. 3A illustrates a link pointer according to one embodiment described herein. Each of the frame list head pointers 234 in periodic frame list 232 is a link pointer 300 that points to a linked list of data structures of various types. In one embodiment, the first field in each data structure, regardless of type, is a link pointer. In one embodiment, link pointers and frame list head pointers have the same defined format. In one embodiment, the format of these link pointers require that referenced data structures be aligned on specific byte boundaries, typically 32 bytes or greater. This alignment requirement leaves the least significant bits of link pointers 300 available for use other than address bits. In one embodiment, a type field 304 may be defined in bits 1 and 2 of a link pointer, and may be a value that represents the type of the next data structure. That is, the type of the data structure pointed to by the link pointer. The various types include transfer descriptors, queue heads, and frame span traversal nodes. In addition, bit 0 may be a terminate bit 306 that is set when the node is the last node in the particular frame. In one embodiment, bits 3 and 4 may be undefined 302. Each data structure, regardless of type, may be referred to as a node.

[0023] Referring again to FIG. 2, the frame list index register 214 is a counter that increments once per millisecond. The contents of frame list index register 214 may be referred to as a frame time. The host controller begins traversal of frame list 232 every 125 microseconds such that traversal begins every micro-frame. There are eight micro-frames in a frame. The result is that the host controller begins traversal of an offset into the frame list eight consecutive times before moving to the next frame list offset. The nodes 236 are linked from the frame list head pointers 234 according to the nearest power of two poll rate such that the longer poll rates appear before the shorter poll rates. Each node may be a transfer descriptor specifying transfer instructions or a transfer related task or a queue head pointing to a queue of transfer related tasks. In order to achieve a poll rate of eight, a node must be linked relative to the frame list so that it is reachable from every eighth offset in the frame list. In order to achieve a poll rate of four, a node must be linked relative to the frame list so that it is reachable from every fourth offset in the frame list. Nodes linked at a specific poll rate may be referred to as level N, where N corresponds to the specific poll rate. For example, level 8, level 4, level 2 and level 1 nodes refer to poll rates 8, 4, 2 and 1, respectively. In this way, each level 8 node (2 ³) must be accomplished eight times every eighth frame; each level 4 node (2 ²) must be accomplished eight times every fourth frame; each level 2 node (2 ¹) must be accomplished eight times every other frame; and each level 1 node (2 ⁰) must be accomplished every micro-frame. As such, the frame list may be viewed as a binary tree structure with the root being the level 1 node and the frame list head pointers being the leaves.

[0024] USB 1.1 defines a 1 millisecond frame, and USB 2.0 superimposes eight 125 microsecond micro-frames over the 1 millisecond USB 1.1 frame. In order to achieve maximum benefit from the 40 times faster signaling rate of high-speed devices added in USB 2.0, USB 2.0 defines a high-speed only host controller and external hubs which isolate the slower USB 1.1 device signaling rate from the high-speed signaling rate supported by USB 2.0 host controllers. Additionally, a split transaction protocol was added to the high-speed USB 2.0 domain. The split transaction protocol is used to forward a transaction via a hub to a full or low-speed device connected below the hub. The split transaction model is a two-phase model that allows a host to start a transaction to a USB 2.0 hub. The USB 2.0 hub will then execute the transaction to the full or low-speed device, and will retain the transaction results. The host controller will then extract the results of the full or low-speed transaction from the USB 2.0 hub during the completion phase of the split transaction. Full and low-speed periodic data streams are managed via a two-stage pipeline in USB 2.0 hubs. This means that it takes several micro-frames to complete a full or low-speed periodic transaction through a USB 2.0 hub. To achieve this, the host controller driver links data structures relative to the frame list so they are reachable at the appropriate poll rate. Based on this construction, common conditions exist where the start of a split transaction will begin in one frame and must complete in the next frame. Because the host controller driver creates a transaction stream with a device as a single data structure or node, in order to support split transactions that span a frame boundary, the node must be reachable from consecutive frame list locations.

[0025] One method to support split transactions that span frame boundaries is to always advance the poll rate to period 1. According to the USB standards, periodic bandwidth must be guaranteed and is managed via a simple additive algorithm. Bandwidth cannot be efficiently managed if all frame-spanning periodic data streams must be promoted to a period 1 poll rate. To increase the efficiency of managing full and low-speed bandwidth, nodes must be made reachable at the normal period rate and at the consecutive frame list location. In one embodiment, to achieve this linkage the host controller may support what will be referred to as two kinds of frame span traversal nodes (FSTN). One kind of FSTN may be referred to as a save place indicator, for example save place indicator 242; and the other may be referred to as a restore place indicator, for example restore place indicator 244. For each save place indicator in a frame's node list, there must be a corresponding restore place indicator in another frame's node list. In one embodiment, the restore place indicator must be in an immediately prior frame.

[0026]FIG. 3B illustrates an embodiment of a frame span traversal node according to one embodiment of the invention. FSTN 310 may comprise two 32 bit pointers, namely, a normal path link pointer 312 and a back path link pointer 314. Each of these pointers have, in one embodiment, the same format as link pointer 300 described above. In one embodiment, as with link pointer 300, the pointers 312 and 314 each may include an undefined field 320 in bits 3 and 4, a type field 316 in bits 1 and 2, and a terminate bit 318 at bit 0.

[0027] When the host controller traverses the nodes of a particular frame, a save place indicator may be encountered. When encountering a save place indicator, the host controller continues traversal at a location specified in the save place indicator's back path link pointer. For example, referring to FIG. 2, when save place indicator 242 is encountered, traversal shifts to transfer descriptor 8C2 as designated by the value in the save place indicator's back path link pointer and as shown by dashed line 252. In addition, when encountering a save place indicator, the host controller stores the save place indicator's normal path link pointer in return node register 216 and sets recovery mode flag 224. Traversal will then continue until a restore place indicator is reached. In one embodiment, when in recovery mode, that is, when the recovery mode flag is set to 1, only nodes that are active queue heads for full or low speed transfer are processed or otherwise executed. When restore place indicator 244 is reached, traversal continues at the location pointed to by the contents of return node register 216.

[0028]FIG. 4 illustrates an example sequence of nodes traversed during one frame according to an embodiment of the invention. When traversing the nodes of Frame C of periodic frame list 232 in FIG. 2, nodes 8C0, 8C1, 8C2, 8C3, 4Y, 2T, <RP>, and 1 are traversed, as shown in FIG. 4. During this traversal, restore place indicator 244 is ignored or passed over by the host controller as indicated by element 400 in FIG. 4. The restore place indicator is passed over because the recovery mode flag 224 has not been set because a save place indicator was not earlier encountered.

[0029]FIG. 5 illustrates an example sequence of nodes traversed during one frame according to an embodiment of the invention. When traversing the nodes of Frame D of periodic frame list 232 in FIG. 2, nodes 8D0, 8D1, 8D2, <SP>, 8C2, 8C3, 4Y, 2T, <RP>, 4Z, 2U, <RP>, and 1 are traversed, as shown in FIG. 5. During this traversal, save place indicator 242 of FIG. 2 is encountered as shown by element 510 of FIG. 5. This shifts traversal from node 8D2 of frame D to node 8C2 of frame C. Traversal continues until restore place indicator 244 of FIG. 2 is encountered, as shown by element 520 of FIG. 5. This shifts traversal from node 2T to node 4Z. Traversal continues from node 2U to node 1, such that when restore place path indicator 244 of FIG. 2 is encountered a second time, it is ignored, as shown by element 530 of FIG. 5.

[0030]FIG. 6 illustrates a flow of actions taken pursuant to an embodiment of the invention. In one embodiment, as discussed above, the location of the periodic frame list is determined by combining the contents of the frame index register and the list base register. In one embodiment, the FCSM may use a current node pointer register and a next node pointer register in traversing and processing a frame list. In another embodiment, local variables may be used. Frame traversal begins, as shown in block 610. The recovery mode flag is cleared, initialized to 0, indicating that the FCSM is not in recovery mode, as shown in block 612. The address of the next node pointer is determined by combining the contents of the frame index register with the contents of the list base register as show in block 614. A check is then made to determine if the next node pointer is the last node in the frame, as shown in block 616. In one embodiment, this may be achieved by looking at the first bit of the next node link pointer, also known as the terminate bit. (See FIGS. 3A and 3B.) If the next node pointer does not point to the last node in the frame, the current node pointer is then set to be the node pointed to by the next node pointer, as shown in block 618. A check is then made to determine if the current node is a save place node and if the recovery mode flag is 0, as shown in block 620. If the recovery mode flag is 0 and the current node is a save place node, the HCSM sets the return node register to be the normal path link pointer of the current node, as shown in block 622. In this way, the return node register maintains a pointer to where traversal will continue when a restore place node is encountered. The recovery mode flag is then set to 1, as shown in block 624. The next node pointer is then set to the value of the back path link pointer of the current node, as shown in block 626. In this way, traversal continues at a node in the prior frame. The HCSM then continues execution at block 616.

[0031] If the current node is not a save place node or if the recovery mode flag is not 0, as shown in block 620, a check is then made to determine whether the current node is a save place node and if the recovery mode flag is set to 1, as shown in block 628. If the current node is a save place node and the HCSM is in recovery mode, as shown in block 628, the save place node is skipped over such that the HCSM sets the next node pointer to be the normal path link pointer of the current node, as shown in block 632. If either the current node is not a save place node or the recovery mode flag is not 1, as shown in block 628, a check is then made to determine whether the current node is a restore place node and the recovery mode flag is 0, as shown in block 630. If the current node is a restore place node and the recovery mode flag is set to 0, the restore place node is skipped over such that the HCSM sets the next node pointer to be the normal path link pointer of the current node, as shown in block 632. The HCSM then continues execution at block 616.

[0032] The HCSM then checks if the current node is a restore place node and the recovery mode flag is set to 1, as shown in block 634. If so, the HCSM takes the recovery path. That is the HCSM sets the recovery mode flag to 0, as shown in block 636, and sets the next node pointer to the pointer in the return node register, as shown in block 638. In this way, the HCSM continues traversal at the node from which traversal was temporarily redirected, which is reachable from the next frame list location. The HCSM then continues execution at block 616.

[0033] If either the current node is not a restore place node or the recovery mode flag is not 1, as shown in block 634, the HCSM processes the current node, as shown in block 640. In view of the earlier checks made by the HCSM, this node must be a “normal” node, that is, neither a save place nor a restore place node, such that the node can be processed according to its type. The HCSM then sets the next node pointer to be the normal path link pointer of the current node, as shown in block 642. The HCSM then continues at block 616.

[0034] If when during traversal of the micro-frames of the current frame, the next node points to the last node in the frame, as shown in block 616, the HCSM transitions to the next frame by waiting for the next frame, as shown in block 644. The HCSM then continues at block 612 with processing the next frame.

[0035] In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 

What is claimed is:
 1. A method comprising: processing a periodic frame list frame by frame, each frame including a list of nodes; traversing each frame node by node; when a save place node is encountered in a first frame, jumping to a destination node pointed to by the save place node, the destination node in a second frame, and continuing the traversing in the second frame at the destination node; when a restore place node is encountered in the second frame, returning to the node after the save place node in the first frame, and continuing the traversing in the first frame.
 2. The method of claim 1 wherein the save place node and the restore place node are each frame spanning traversal nodes which comprise a back path link pointer and a normal path link pointer.
 3. The method of claim 2 wherein jumping comprises: checking whether a recovery mode flag is set; if the recovery mode flag is not set, storing a back path link pointer of the save place node in a return node register, setting the recovery mode flag, and continuing the traversing at the node pointed to by the normal path link pointer of the save place node.
 4. The method of claim 2 wherein returning comprises: checking whether a recovery mode flag is set; if the recovery mode flag is set, clearing the recovery mode flag and continuing the traversing at the node pointed to by a return node register.
 5. A method comprising: processing a plurality of external bus data including traversing a plurality of nodes in a frame of a plurality of frames in a periodic frame list; when a save place node is encountered, checking whether a recovery mode flag is set, if the recovery mode flag is set then continuing the traversing with a next node, if the recovery mode flag is not set then storing a back path link pointer of the save place node in a return node register, setting the recovery mode flag, and continuing the traversing at the node pointed to by a normal path link pointer of the save place node; when a restore place node is reached, checking whether the recovery mode flag is set, if the recovery mode flag is set then clearing the recovery mode flag and continuing the traversing at the node pointed to by the contents of the return node register, if the recovery mode flag is not set then continuing the traversing with the next node.
 6. The method of claim 5 wherein the periodic frame list comprises a plurality of frame head pointers each pointing to a node list of some of the plurality of nodes.
 7. The method of claim 6 wherein the nodes comprise a binary tree structure.
 8. The method of claim 5 wherein the external bus data is information regarding at least one device adhering to a Universal Serial Bus standard.
 9. A system comprising: a processor and a memory coupled to an internal bus; an external bus controller coupled to the internal bus, the external bus controller having a plurality of registers including a return node address register, and having a plurality of flags including a recovery mode flag; a state machine included with the external bus controller, the state machine to process a plurality of external bus data in the memory by traversing a periodic frame list including a plurality of frames, each frame having a plurality of nodes, the periodic frame list including at least one save place node and at least one restore place node.
 10. The system of claim 9 wherein the external bus controller provides support for a well known external bus standard.
 11. The system of claim 10 wherein the well known external bus standard is the Universal Serial Bus ver 2.0 standard.
 12. A machine readable medium having instructions stored thereon which when executed by a processor cause the processor to perform operations comprising: processing a periodic frame list frame by frame, each frame including a list of nodes; traversing each frame node by node; when a save place node is reached in a first frame, jumping to a destination node pointed to by the save place node in a second frame and continuing the traversing in the second frame at the destination node; when a restore place node is reached in the second frame, returning to the node after the save place node in the first frame and continuing the traversing in the first frame.
 13. The machine readable medium of claim 12 wherein the save place node and the restore place node are each frame spanning traversal nodes which comprise a back path link pointer and a normal path link pointer.
 14. The machine readable medium of claim 13 wherein jumping comprises: checking whether a recovery mode flag is set; if the recovery mode flag is not set, storing a back path link pointer of the save place node in a return node register, setting the recovery mode flag, and continuing the traversing at the node pointed to by the normal path link pointer of the save place node.
 15. The machine readable medium of claim 13 wherein returning comprises: checking whether a recovery mode flag is set; if the recovery mode flag is set, clearing the recovery mode flag and continuing the traversing at the node pointed to by a return node register. 